1. Field of the Invention
The present invention relates to a method and apparatus for reducing noise in electronic devices, and more particularly, to a method and apparatus for reducing transient noise generated when electronic devices are switched to drive high output currents.
2. Description of the Prior Art
In many electronic devices and systems, the presence or generation of transient noise is an undesireable consequence of normal operation. although low levels of transient noise may be tolerated under certain operating conditions, transient noise of high levels may result in inaccurate outputs for the electronic devices, or may cause the electronic devices to exceed their operating conditions for a period of time. This may lead to the ultimate failure of the electronic devices and the systems in which they are utilized.
FIG. 1 illustrates an output driver 10 used in a conventional integrated circuit. The output driver 10 is activated by an input voltage V.sub.IN received from the logic of the integrated circuit (not shown) and provides an output voltage V.sub.OUT used to drive external electronic device. The output driver 10 illustrated includes a parasitic inductance 12 representative of the inductance of the integrated circuit, a first switch 14 and a second switch 16.
Prior to activation of the output driver 10, the first switch 14 is closed and the second switch 16 is open. A power supply voltage V.sub.DD is applied to the load capacitance 18 through the first switch 14, allowing the load capacitance 18 to become fully charged, maintaining the output voltage V.sub.OUT equal to the power supply voltage V.sub.DD. The parasitic inductance 12 of the integrated circuit is electrically isolated from the power supply voltage V.sub.DD and the load capacitance 18 by the open second switch 16. As illustrated in FIG. 2, the current level I.sub.G passing through the parasitic inductance 12 is therefore initially at a low level. Similarly, the voltage level across the parasitic inductance 12 is also minimal prior to the closing of the switch 16.
At a time which may be referred to as time zero (t.sub.0), the input voltage V.sub.IN changes state to activate the output driver 10. In response, the first switch 14 begins to open and the second switch 16 begins to close. The fully charged load capacitance 18 therefore begins to discharge very quickly, supplying a current to I.sub.0 to the parasitic inductance 12. The level of current I.sub.G passing through the parasitic inductance 12 therefore increases rapidly as a result of the application across the inductance of a voltage approximately a step function at time t.sub.0, as illustrated in FIG. 2. Similarly, the output voltage V.sub.OUT commences to decrease as the load capacitance 18 discharges over time, as illustrated in FIG. 3.
FIG. 4 illustrates the rate of change of the current I.sub.G passing through the parasitic inductance 12. Due to the rapid increase in the current I.sub.G which occurs at time t.sub.0, a spike 19 is created in the rate of change of the current I.sub.G. Since noise is proportional to the rate of change of the current I.sub.G passing through the parasitic inductance 12, the spike 19 results in the undesireable generation of a high level of noise.
In recent years, methods and devices have been sought to drive higher and higher values of load capacitance 18. When larger values of load capacitance 18 are made to discharge within the same time period as conventional values of capacitance, higher rates of change of current are experienced through the parasitic inductance 12 electronic device. As a result, even higher levels of transient noise are generated than exit in conventional output drivers.
In addition, faster circuit switching has recently been sought. However, when conventional values of load capacitance are driven within a shorter period of time, higher levels of transient noise are also produced.
Conventional electronic devices have therefore been limited to high speed, high noise applications or low speed, low noise applications.
Some conventional electronic devices have also required the consumption of DC, increasing the total power drain caused by the operation of the electronic devices.